Axial flow gas turbine engines, such as those utilized in commercial aircraft, typically include a compressor that draws air in and compresses the air. The compressed air is passed along a primary flow path to a combustor, where the air is mixed with fuel and ignited. The resultant gasses are expelled along an annular flow path into a turbine section, where the resultant gasses expand and drive the turbine section to rotate. The initial air, the air fuel mixture, and the gasses resultant from combustion in the combustor are referred to as a working fluid. During operation, the working fluid travels along an annular flow path that extends axially through the compressor, combustor and turbine sections. In typical example engines, a stator assembly extends about the annular flow path for confining the working fluid to the flow path and for directing the fluid along the flow path.
Multiple rotor assemblies are positioned in the turbine section and each assembly includes multiple rotor blades which extend radially outward across the flow path. A blade outer air seal (BOAS) circumscribes the rotor blades of each rotor assembly and confines the working fluid to the flow path. The blade outer air seal further maintains a proper rotor tip clearance between the rotor tip and the outer diameter of the annular flow path.
The rotor assemblies are positioned in a hot gas environment within the flow path, and the corresponding blade outer air seals use forced cooling in order to prevent heat damage to the blade outer air seal. Most modern blade outer air seals use either a refractory metal core (RMC) microcircuit, a refractory metal core super convective shroud (RMC SCS), or conventional super convective shroud combined with impingement or film cooling techniques to provide the forced cooling.
Refractory metal core cooling utilizes cooling passages internal to the blade outer air seal, with the internal passages including inlet holes, exit holes and trip strips on a hot side of the internal passage. The hot side of the internal passage is the side of the passage facing the annular flow path.
The refractory metal cores are a cast component. In order to achieve two or more supporting points for each channel during the casting, and thereby ensure proper casting, at least one connector is used at the boundary between each channel and a neighboring channel. As a result of the connectors, openings referred to as “cross passages” are created in the walls separating the channels. During operation, however, the internal pressures within each channel are different from the internal pressure of the neighboring channels. This pressure variation leads to cross channel leakage through the cross passages. Cross channel leakage reduces convection efficiency of the blade outer air seal, causing the blade outer air seal to require a higher overall cooling flow rate, or resulting in a reduced lifespan of the blade outer air seal.